Method and apparatus for a differential feedback in an active impedence feedback circuit

ABSTRACT

A method and apparatus is provided for performing an active impedance differential current feedback. A signal is received. A current feedback of an output signal in an active feedback synthesis mode is performed based upon the signal. The active current feedback includes using an active feedback network for conditioning an output signal, converting the signal into a differential current signal, and summing at least two components of the differential current signal for performing a differential feedback.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to telecommunications, and, moreparticularly, to providing a differential current feedback in an activeimpedance feedback circuit for signal reception and/or transmission.

[0003] 2. Description of the Related Art

[0004] In communications systems, particularly telephony such as a PlainOld Telephone System (POTS), it is common practice to transmit signalsbetween a subscriber station and a central switching office via atwo-wire, bi-directional communication channel. A line card generallyconnects the subscriber station to the central switching office. Thefunctions of the line card include supplying talk battery, performingwake-up sequences of circuits to allow communications to take place, andthe like. Voltage signals are processed and conditioned when beingdriven onto telecommunication lines.

[0005] POTS was designed primarily for voice communication, and thusprovides an inadequate data transmission rate for many modernapplications. To meet the demand for high-speed communication, designershave sought innovative and cost-effective solutions that would takeadvantage of the existing network infrastructure. Several technologicalsolutions proposed in the telecommunications industry use the existingnetwork of telephone wires. A promising one of these technologies is theDigital Subscriber Line (xDSL or DSL) technology.

[0006] xDSL is making the existing network of telephone lines morerobust and versatile. Once considered virtually unusable for broadbandcommunications, an ordinary twisted pair equipped with DSL interfacescan transmit video, television, and very high-speed data. The fact thatmore than six hundred million telephone lines exist around the world isa compelling reason for these lines to be used as the primarytransmission conduits for at least several more decades. Because DSLutilizes telephone wiring already installed in virtually every home andbusiness in the world, it has been embraced by many as one of the morepromising and viable options.

[0007] There are now at least three popular versions of DSL technology,namely Asymmetrical Digital Subscriber Line (ADSL), Very High-SpeedDigital Subscriber Line (VDSL), and Symmetric Digital Subscriber Line(SDSL). Although each technology is generally directed at differenttypes of users, they all share certain characteristics. For example, allfour DSL systems utilize the existing, ubiquitous telephone wiringinfrastructure, deliver greater bandwidth, and operate by employingspecial digital signal processing. Because the aforementionedtechnologies are well known in the art, they will not be described indetail herein.

[0008] DSL and POTS technologies can co-exist in one line (e.g., alsoreferred to as a “subscriber line”). Traditional analog voice bandinterfaces use the same frequency band, 0-4 Kilohertz (KHz), astelephone service, thereby preventing concurrent voice and data use. ADSL interface, on the other hand, operates at frequencies above thevoice channels, from 25 KHz to 1.1 Megahertz (MHz). Thus, a single DSLline is capable of offering simultaneous channels for voice and data. Itshould be noted that the standards for certain derivatives of ADSL arestill in definition as of this writing, and therefore are subject tochange. DSL systems use digital signal processing (DSP) to increasethroughput and signal quality through common copper telephone wire. Itprovides a downstream data transfer rate from the DSL Point-of-Presence(POP) to the subscriber location at speeds of up to 1.5 megabits persecond (MBPS). The transfer rate of 1.5 MBPS, for instance, is fiftytimes faster than a conventional 28.8 kilobits per second (KBPS)transfer rate typically found in conventional POTS systems.

[0009] DSL systems generally employ a signal detection system thatmonitors the telephone line for communication requests. Morespecifically, the line card in the central office polls the telephoneline to detect any communication requests from a DSL data transceiver,such as a DSL modem, located at a subscriber station. There are multipletypes of signals that are received and transmitted over multiple signalpaths during telecommunication operation. Many times, feedbackconfigurations in the amplifiers that process the transmission signalscause noise and power problems.

[0010] Many of today's amplifier circuits may call for electroniccomponents that have high bandwidth, which may increase consumption ofpower. Many times, power consumption in the line card can becomeundesirably high. Amplifier circuits that are used to conditioncommunication signals often consume large amounts of power. Excessivepower use can compromise the effectiveness of line cards, particularlyfor remote line cards, which rely upon portable power supplies.Excessive power consumption can also require additional resources tocounteract the effects of high power consumption, such as additionalcooling systems to keep line card circuitry in operating condition.Excessive power consumption can also require additional circuits tofurnish the required amounts of power needed for efficient operation ofline cards. Excessive power consumption can cause appreciableinefficiencies in the operation of line cards and the communicationsystem as a whole.

[0011] The prior art implementations of signal conditioning circuitsgenerally implement signal feedback configurations that generally takethe output voltage signal and then feed the voltage signal back to anegative input of an amplifier within a circuit. In other words, thedirect output voltage signal is the feedback signal used in theimplementations described above. Among the problems associated with thecurrent implementations, include the fact that a larger signal is fedback into the circuit described above. The problem with such animplementation is that larger signals may generally carry larger amountsof noise. Therefore, feeding back larger signals amounts to feeding backlarger amounts of noise into the circuit, this may cause performanceproblems in the amplifier circuit.

[0012] Additionally, feeding back the output voltage signal may requireamplifiers that have relatively large bandwidth capabilities. Utilizingamplifiers with larger bandwidth capabilities generally increases powerconsumption. Additionally, many prior art systems employ feedbackconfigurations that use single-ended feedback signals. The circuits thatuse these types of configurations may experience an excessive amount oflongitudinal signals, such as longitudinal currents. Longitudinalsignals may enter one of more amplification stages within a circuit andcause excessive noise. Utilizing the current methodologies, theperformance of a signal conditioning circuit may be compromised.

[0013] The present invention is directed to overcoming, or at leastreducing the effects of, one or more of the problems set forth above.

SUMMARY OF THE INVENTION

[0014] In one aspect of the present invention, a method is provided forperforming an active impedance differential current feedback. A signalis received. A current feedback of an output signal in an activefeedback synthesis mode is performed based upon the signal. The activecurrent feedback includes using an active feedback network forconditioning an output signal, converting the signal into a differentialcurrent signal, and summing at least two components of the differentialcurrent signal for performing a differential feedback.

[0015] In another aspect of the present invention, an apparatus isprovided for performing an active impedance differential currentfeedback. The apparatus of the present invention includes a firstamplifier and a second amplifier to buffer a differential input signalto generate a differential output signal. The apparatus also includes anactive impedance network that is adapted to condition the differentialoutput signal and sum at a first component and a second component of afirst portion of the conditioned differential output signal. The activeimpedance network is also adapted to and to sum first and secondcomponents of a second portion of the conditioned differential outputsignal for generating a differential feedback signal for feedback intothe first and second amplifiers.

[0016] In another aspect of the present invention, an apparatus isprovided for performing an active impedance differential currentfeedback. The apparatus of the present invention includes avoltage-to-current signal converter to convert a differential inputvoltage signal and a first buffer amplifier operatively coupled to thevoltage to current signal converter. The first buffer amplifier isadapted to receive a component of a converted differential input signalon a first input terminal and produce a first buffered current signal.The apparatus also includes a first sense resistor operatively coupledwith the buffered amplifier and a second buffer amplifier operativelycoupled to the voltage-to-current signal converter. The second bufferamplifier is adapted to receive a component of the converteddifferential input signal on a second input terminal and produce asecond buffered current signal. The apparatus also includes a secondsense resistor operatively coupled with the second buffer amplifier; anactive feedback impedance network operatively coupled with the first andsecond sense resistors; and a first datasense amplifier operativelycoupled to the active feedback impedance network and to the first senseresistor. The first datasense amplifier is adapted to generate a firstand second component of a differential feedback signal. The apparatusalso includes a second datasense amplifier operatively coupled to theactive feedback impedance network and to the second sense resistor. Thesecond datasense amplifier is adapted to generate a third and fourthcomponent of the differential feedback signal. The apparatus alsoincludes a first summing node for summing the first and secondcomponents of the differential feedback signal to generate a firstcomponent of a summed differential feedback signal for feedback into thefirst buffer amplifier and a second summing node for summing the thirdand fourth components of the differential feedback signal to generate asecond component of the summed differential feedback signal for feedbackinto the second buffer amplifier.

[0017] In another aspect of the present invention, a system is providedfor performing an active impedance differential current feedback. Thesystem of the present invention comprises a subscriber line. The systemalso includes a line card electronically coupled with the subscriberline. The line card is adapted to receiving a signal. The line card isalso capable of performing a current feedback of an output signal in anactive feedback synthesis mode based upon the signal. Performing theactive current feedback includes using an active feedback network forconditioning an output signal, converting the signal into a differentialcurrent signal, and summing at least two components of the differentialcurrent signal for performing a differential feedback.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention may be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich like reference numerals identify like elements, and in which:

[0019]FIG. 1 illustrates a first embodiment of an apparatus inaccordance with one illustrative embodiment of the present invention;

[0020]FIG. 2 illustrates an implementation of a line card into theapparatus described in FIG. 1 in accordance with one illustrativeembodiment of the present invention;

[0021]FIG. 3 illustrates a more detailed depiction of the line card inaccordance with one illustrative embodiment of the present invention;

[0022]FIG. 4 illustrates a simplified block diagram depiction of theSLIC described in FIG. 3, in accordance with one illustrative embodimentof the present invention;

[0023]FIG. 5 illustrates a block diagram depiction of a signalconditioning unit in the SLIC of FIG. 4, in accordance with oneillustrative embodiment of the present invention;

[0024]FIG. 6 illustrates a more detailed block diagram depiction of theSLIC of FIG. 4, in accordance with one illustrative embodiment of thepresent invention;

[0025]FIG. 7 illustrates a circuit diagram of the signal amplifier inthe SLIC of FIG. 4, in accordance with one illustrative embodiment ofthe present invention; and

[0026]FIG. 9 illustrates a flowchart depiction of a method in accordancewith one illustrative embodiment of the present invention.

[0027] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the invention to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

[0028] Illustrative embodiments of the invention are described below. Inthe interest of clarity, not all features of an actual implementationare described in this specification. It will of course be appreciatedthat in the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure.

[0029] Embodiments of the present invention provide for a method andapparatus for utilizing a current feedback configuration to reduce noiseand/or power consumption in an amplifier circuit used to conditionelectrical signals (e.g., a communications signal). Embodiments of thepresent invention provide for feeding back a smaller current signalrelative to an output signal in a signal amplification/buffer circuitry.Embodiments of the present invention call for canceling the feedbackcurrent signal and using the cancelled signal as a feedback in an activeimpedance loop. The active impedance loop is capable of reacting to anoutput impedance seen by an amplifier circuit and reacting to theimpedance to cancel a substantial portion of the feedback signal usingan active impedance feedback circuitry to feedback a smaller signal.Therefore, a lower amount of noise may be experienced by a signalconditioning circuit and improvements in power consumption may berealized. Utilizing a differential feedback configuration, longitudinalbalance in a signal conditioning circuit may be achieved.

[0030] An impedance circuitry that comprises an impedance unit that isproportional to an output impedance seen by a signal conditioningcircuit, along with one or more sense resistors that are proportional tothe output impedance, may be used to perform an active impedancefeedback. For example, embodiments of the present invention call fordetermining a turns ratio N of an output impedance (e.g. a transformer)and implementing an active impedance feedback implementation that isproportional to N. An output voltage signal is converted into a currentsignal in a feedback loop. Utilizing the active impedance circuit, afeedback current signal is summed such that the feedback current signalis cancelled as it is being fed back into an input buffer or amplifier.Therefore, better noise-to-signal ratio may be achieved. Also, thedynamic range of the signal conditioning circuit may be improved.Embodiments of the present invention also call for converting the outputsignal into a current signal and summing the current signals such that asmall signal (i.e., a cancelled current signal) is sent back into theinput buffer/amplifier as a feedback signal. Lower bandwidth amplifiersmay be implemented due to the operation in the current domain.Therefore, the amplifier experiences a lower gain, thereby reducingpower consumption. The current feedback configuration may be used tobroadband the amplifiers in a signal conditioning circuit. Further,using the current feedback in a differential feedback configuration mayprovide the added benefit of promoting longitudinal balance. Althoughembodiments of the present invention are described in the context of aline card implementation, the teachings provided by the invention may beimplemented into a variety of signal conditioning circuits in a varietyof electronic/electrical applications.

[0031] Referring now to the drawings and in particular to FIG. 1, anapparatus 100 in accordance with the present invention is illustrated.The apparatus 100 includes a central office 110 that is coupled to asubscriber station 120 via a subscriber line 130. The central office 110and the subscriber station 120 are capable of sending and receiving asignal comprising a voice and data band. The voice band, as used herein,refers to a POTS voice signal ranging from 0-4 KHz. The data band refersto frequencies above the voice band, and may include, for example, thefrequency range employed in xDSL technologies. In one embodiment, thesubscriber line 130 may be a Public Switched Telephone Network (PSTN)line, a Private Branch Exchange (PBX) line, or any other medium capableof transmitting signals.

[0032] The subscriber station 120 may be a telephonic device capable ofsupporting pulse dialing. The term “telephonic device,” as utilizedherein, includes a telephone, or any other device capable of providing acommunication link between at least two users. In one embodiment, thesubscriber station 120 may be one of a variety of available conventionaltelephones, such as wired telephones and similar devices. In analternative embodiment, the subscriber station 120 may be any “device”capable of performing a substantially equivalent function of aconventional telephone, which may include, but is not limited to,transmitting and/or receiving voice and data signals. Examples of thesubscriber station 120 include a data processing system (DPS) utilizinga modem to perform telephony, a television phone, a wireless local loop,a DPS working in conjunction with a telephone, Internet Protocol (IP)telephony, and the like. IP telephony is a general term for thetechnologies that use the Internet Protocol's packet-switchedconnections to exchange voice, fax, and other forms of information thathave traditionally been carried over the dedicated circuit-switchedconnections of the public switched telephone network (PSTN). One exampleof IP telephony is an Internet Phone, a software program that runs on aDPS and simulates a conventional phone, allowing an end user to speakthrough a microphone and hear through DPS speakers. The calls travelover the Internet as packets of data on shared lines, avoiding the tollsof the PSTN.

[0033] Turning now to FIG. 2, a line card 210 and a DSL modem 220 areillustrated in accordance with the present invention. In one embodiment,the line card 210, which is integrated into the central office 110, iscoupled with the DSL modem 220, which resides within the subscriberstation 120. Because voice and/or data can be transmitted on thesubscriber line 130, the signal received and transmitted by the linecard 210 and the DSL modem 220 may include voice and data bandfrequencies.

[0034] The line card 210 may be located at a central office 110 or aremote location somewhere between the central office 110 and thesubscriber station 120 (see FIG. 1). The line card 210 services thesubscriber station 120, which in the illustrated embodiment is atelephonic device. The line card 210 is capable of processing DC voltagesignals and AC signals. The subscriber line 130 in the instantembodiment is a telephone line. The combination of the telephone device(subscriber station 120) and the telephone line (subscriber line 130) isgenerally referred to as a subscriber loop.

[0035] The line card 210, which may be capable of supporting a pluralityof subscriber lines 130, performs, among other things, two fundamentalfunctions: DC loop supervision and DC feed. The purpose of DC feed is tosupply enough power to operate the subscriber station 120 at thecustomer end. The purpose of DC loop supervision is to detect changes inDC load, such as on-hook events, off-hook events, rotary dialing, or anyother event that cause the DC load to change. In the interest of clarityand to avoid obscuring the invention, only that portion of the line card210 that is helpful to an understanding of the invention is illustrated.

[0036] Turning now to FIG. 3, one embodiment of the line card 210 isillustrated. In one embodiment, the line card 210 comprises a subscriberline interface circuit (SLIC) 310 as well as a subscriber lineaudio-processing circuit (SLAC) 320. The SLIC 310 performs a variety ofinterface functions between the line card 210 and the subscriber line130. The SLIC 310 is also capable of performing a variety of functions,such as battery feed, overload protection, polarity reversal, on-hooktransmission, and current limiting. The SLIC 310 is connected to theSLAC 320. The SLAC 320 is capable of processing analog-to-digital (A/D)and digital-to-analog (D/A) signal conversion, filtering, feed control,and supervision.

[0037] Turning now to FIG. 4, a more detailed description of the linecard 210 in accordance with one embodiment of the present invention isillustrated. In one embodiment, the SLIC 310 comprises a signalamplifier 410. The signal amplifier 410 is capable of amplifying anoutput signal sent by the line card 210. The signal amplifier 410provides for processing and amplifying the communication signal in sucha way that reduced power and lower noise levels are achieved. The signalamplifier 410 also receives communication signals, buffers them, andprovides an output buffering stage for driving the receivedcommunication signals. The signal amplifier 410 comprises a differentialactive feedback circuit 420 that is capable of providing an activeimpedance feedback described above in a differential feedbackconfiguration. A more detailed description of the circuitry relating tothe differential active feedback circuit 420 is provided in the drawingsand related description below.

[0038] Turning now to FIG. 5, a block diagram representation of oneembodiment of the signal amplifier 410 is illustrated. The signalamplifier 410 comprises a set of input buffers 510. The input buffers510 receive a differential input signal that comprises an input positiveportion of a line 515 (input_pos) and an input negative portion on aline 517 (input_neg). The input buffers 510 buffer the differentialinput signals and process them using a signal conditioning unit 530,which provides a differential output signal to a load impedance 550. Theload impedance 550 may comprise an equivalent impedance of atransmission line, which may be experienced through a transformer,affecting the output impedance seen by the signal conditioning unit 530.

[0039] The signal conditioning unit 530 receives a differential feedbacksignal pair 560, which is fed-back differentially from the signal thatis sent to the load. The differential feedback signal pair 560 isgenerally current signals. The signal conditioning unit 530 comprises avoltage-to-current conversion unit 570, which converts the differentialvoltage pair 560 into a differential current signal. In one embodiment,the signal condition unit 530 sums components of the differentialcurrent signal for performing cancellation, thereby promotinglongitudinal balance. The signal conditioning unit 530 may also comprisean active feedback network 540. The active feedback network 540 iscapable of extracting the differential voltage signal from theconversion unit 570 and providing a differential active feedbacksynthesis such that a small differential signal pair 570 is fed-backinto an amplifier, which may be housed within the input buffers 510. Amore detailed description of the various circuits relating to thecomponents shown in FIG. 5 is provided in greater detail below.

[0040] Turning now to FIG. 6, a block diagram representation of theimplementation of the signal amplifier 410 described in the presentinvention is illustrated. The block diagram illustrated in FIG. 6generally illustrates the processing of communication signals receivedand transmitted by the line card 210 using the signal amplifier 410 inthe SLIC 310. In one embodiment, the block diagram of FIG. 6 may beimplemented in a semiconductor device (e.g., a semiconductor chip).

[0041] The downstream (DS) signal voltage (downstream relative to theline card 210) is presented at the DDWN+/−pins (on a line 610 a and aline 610 b), where it is converted into a differential current by adatadown block 620, which drives a set of output buffers 621. In oneembodiment, the output buffers 621 have inverting inputs. Generally, theoutput buffers 621 are configured as transconductance amplifiers,therefore, the DS signal is re-converted into a differential voltage atoutput pins AY 630 a and BY 630 b. In one embodiment, the gain appliedto the DS signal may provide a signal that may be approximately 48 voltspeak to peak from DDWN+/−610 a, 610 b to AY 630 a and BY 630 b. Theoutput voltage at AY, BY 630 a, 630 b drives a step-down transformer635, which has a turns ratio of N:1, through a pair of current sensingresistors, Rs 632 a, 632 b.

[0042] A datasense block 640 implements the communications signal path.Two transconductance stages within the datasense block 640 convert thevoltage signal across AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643 into adifferential current signal pair, DUP+645 a and DUP-645 b. External loadresistors 648 a, 648 b convert the signal back to a signal voltage. Asthe voltage signal across AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643becomes proportional to the loop current (including the turns ratio, N),the transfer function from loop current to the differential currentsignal pair DUP+645 a and DUP-645 b becomes a current gain. The currentgain may be fixed for a given set of external components.

[0043] The circuit comprises a pair of sense resistors (NRs 650 a andNRs 650 b) that are proportional to the turns ratio N, and an activeimpedance NZL 750 to provide an approximate cancellation of the DSsignal across AD, ABAL 638 a, 642 and BD, BBAL 638 b, 643. Thecancellation of the DS signal allows the datasense block 640 to operatewithout having to process the full DS signal. This significantly reducesthe dynamic range requirements of the datasense block 640, which in oneembodiment, may be designed to cope with a substantially worst-casecancellation of 12 dB.

[0044] Additional current outputs from the datasense block 640 may drivefeedback signal currents into summing nodes of the output buffers 621blocks. This forms a feedback loop, which sets the terminating impedanceacross AD 638 a to BD 638 b. In one embodiment, provided that Rs 632 a,632 b is substantially made equal to N²*6.2 ohms, the impedance at AD638 a, BD 638 b may be N²*100 ohms. Since the feedback loop describedabove generally does not respond to DS signals, it does not control theterminating impedance for DS signals. Generally, the impedance may below in this case. However, less than ideal echo cancellation may resultin some variability of the DS voltage gain.

[0045] The circuitry shown in FIG. 6 comprises an active feedbacknetwork 540. The active feedback network 540 is capable of providing anactive adjustment to the impedance provided by the transformer 635 suchthat a cancelled signal is fed back via the A balance and the B balanceterminals 642, 643. The node 646 (node G) is created by summing theoutputs from the datasense 640. The summing of the signals on node G646, which is fed back to the output buffers 621 provides a cancelledsignal feedback, which is a signal that is smaller than what it would bewithout cancellation. The components of the active feedback network 540is based upon the turn ratio N of the transformer 635. In other words,the value of the components of the active feedback network 540 isproportional to the turns ratio N. In one embodiment, the number N is alarge number, such as 100. Therefore, if a load is 100 times bigger thanthe components of the active feedback network 540, the operating poweris only affected by 1%. Hence, using the active feedback network 540,the feedback signal is converted into a current domain and then summedon the node G 646, such that the signal that is fed back to theamplifiers are essentially cancelled and very small. A more detailedimplementation of the active feedback network 540 is provided in FIG. 7and accompanying description below.

[0046] Turning now to FIG. 7, one embodiment of a circuit implementingthe active feedback network 540 in accordance with embodiments of thepresent invention is illustrated. A differential input voltage input_posand input_neg 515, 517 are provided to a buffer 710. The buffer 710 mayconvert the received input voltage signal into a current signal. Theoutput of the buffer on a line 715 is then fed to an amplifier 720 witha negative feedback configuration. An output terminal of the amplifier720 is coupled with a terminal of the sense resistor 632 a. The secondterminal of the sense resistor 632 a (node B) is then fed back to avoltage current amplifier located within the datasense 640, which maycomprise amplifiers 640 a and 640 b). Furthermore, the amplifier 640 aand 640 b may comprise one of more amplifiers and/or buffers built ineach one of them. The amplifier 640 a then provides an output feedback,which may be the positive terminal of a differential output feedbacksignal. The sense resistor 632 b provides a node D that is coupled to aninput of a voltage-to-current amplifier 640 a within the datasense 640.The outputs of the amplifiers 640 a, 640 a are then provided to theterminals 645 a, 645 b (shown, in FIG. 6). An output of the datasensesignal 640 a, which forms a node G (745), is summed with an output (nodeE) coming from the voltage-to-current signal amplifiers 640 a and 640 a.The summation of the signals on node G (745) is then fed back to theamplifier 720 as a current signal.

[0047] The active feedback network 540 provides a signal on a node A(755) and a node C (757) to the datasense voltage-to-current amplifiers640 a, 640 b, respectively. A terminal of the resistor R_(S) 632 a iscoupled with a terminal of a first resistor 730 (N*R_(S)) that isproportional to the sense resistor 632 a by a factor of the turns ratioN of the transformer 635 (shown in FIG. 6). Similarly, a second resistor740 (N*R_(S)) is also proportional to the resistor R_(S) 632 b by afactor of N. Additionally, the active feedback network 540 comprises animpedance device that provides an active impedance 750 (NZ′_(L)), whichis proportional to the output impedance load Z_(L) 760. The activeimpedance 750 is proportional to the impedance load 760 by a factor ofN. The second terminal of the resistor 730 is coupled with an inputterminal of the active impedance 750, forming the node A 755. Similarly,the second terminal of the resistor 740 is coupled with the secondterminal of the impedance 750 forming node C 757. Node A 755 and node C757 are fed into the datasense voltage-to-current amplifiers 640 a, 640b. A first output of the datasense amplifiers 640 a and 640 b are summedon the node G 745 and fed back to the amplifier 720 via the node F 747,which is coupled to the line 715. In one embodiment, the voltage atnodes A and B, (755 and 717) are substantially the same. Additionally,the voltage at node C (757) and D (759) are also substantially the same.

[0048] The feedback that is sent to the amplifier 720 is a summation ofthe signal at node A 755 and the negative version of the signal on nodeB 717. In other words, the feedback signal, which is denoted by thesignal I_(pos) on the node G 745, is the signal on node A 755 minus thesignal on node B 717. The feedback signal (node A 755 minus node B 717)is substantially small, i.e., very close to zero. Similarly a secondoutput from the voltage-to-current amplifiers 640 a, 640 b, may besummed on a node H (coupled to a line 749) and fed-back to an amplifier790, whose other input may be a reference voltage. The feedback signal,which is denoted by the signal I_(neg) on the node H 749, is the signalon node C 757 minus the signal on node D 759. The feedback signal (nodeC 757 minus node D 759) is substantially small, i.e., very close tozero. This feedback level is achieved if the circuit illustrated in FIG.7 is properly balanced.

[0049] One factor that leads to the proper balancing of the circuit inFIG. 7 is a function on how accurate is the proportional impedance 750along with resistors 730 and 740. When the active feedback network 540is set at correct proportional levels, and balance of the circuit isachieved, the feedback signal is substantially zero, thereby providingfor smaller noise effects and the other benefits described above. Inother words, the feedback provided by the circuit illustrated in FIG. 7provides for a cancelled signal as a feedback signal for improvedoperation of the circuit.

[0050] By utilizing the small signals, the dynamic range of theamplifiers of FIG. 7 may be reduced, therefore, lower power amplifiersare implemented reducing power, and less noise is experienced. One ofthe advantages of manipulating the voltage signals and utilizing them ascurrent signals by the circuit in FIG. 7 occurs because from a voltagepoint of view, the circuit “sees” a low gain and the impedance of thecurrent source are very high. Therefore, the gain of the amplifier isnear unity from the voltage feedback point of view. Hence, employing theelectrical scheme described above, an amplifier will typically have asingle pole roll-off. In order to keep the amplifier stable, since it isdifficult to determine the actual gain, compensation for a unit gain isimplemented; wide bandwidth may be achieved by putting different amountsof compensation in the amplifiers of the circuit in FIG. 7. Utilizingthe current implementation, we are actually taking some of the currentfrom the output of the circuit and summing it into the feedback signal,which provides for better matching and balance on the two sides of theimpedance loop. Therefore, employing embodiments of the presentinvention, reduced dynamic range, power, and noise advantages areachieved, therefore, smaller circuits may be used, and savings in powerand space realized.

[0051] Turning now to FIG. 8, a flow chart illustration of embodimentsof the present invention is illustrated. Upon receiving the signal, thesignal amplifier 410 converts the output voltage into a current signalfor manipulation of a current signal to reduce the dynamic range of theamplifiers being implemented. The signal amplifier 410 also conditionsthe received signal, which is in a differential feedback format, usingan active feedback network 540 (block 820). The feedback signal may thenbe converted to a differential current signal for summation andcancellation (block 830). The active feedback network 540 provides foran active feedback synthesis to provide a summation of the feedbacksignal such that it is reduced substantially, and is fed back to anamplifier in the circuit (block 840). The conditioned differentialfeedback signal is then fed-back into the amplifier providing for lessnoise amplification and more efficient processing of the receivedsignal. Utilizing embodiments of the present invention, a more robust,lower power, lower noise signal amplifier may be achieved. Therefore, amore efficient SLIC 310 may be used using embodiments described in thepresent invention. Therefore, lower line cards 210 may be realized.

[0052] Although for illustrative purposes, embodiments of the presentinvention have been discussed in the context of a line cardsapplication, the amplifier arrangements taught by embodiments of thepresent invention is not limited to line card applications. The conceptstaught by embodiments of the present invention may be utilized in avariety of electronic applications. The apparatuses 110, 120, 130 may beintegrated in a system capable of transmitting and receiving signalshaving a voice band and/or a data band. The teachings of the presentinvention may be implemented in a line card 210 that supports POTStechnology, ADSL technology, and/or similar technologies. The teachingsof the present invention may also be implemented in various otherelectronics applications.

[0053] The particular embodiments disclosed above are illustrative only,as the invention may be modified and practiced in different butequivalent manners apparent to those skilled in the art having thebenefit of the teachings herein. Furthermore, no limitations areintended to the details of construction or design herein shown, otherthan as described in the claims below. It is therefore evident that theparticular embodiments disclosed above may be altered or modified andall such variations are considered within the scope and spirit of theinvention. Accordingly, the protection sought herein is as set forth inthe claims below.

What is claimed:
 1. A method, comprising: receiving a signal; andperforming a current feedback of an output signal in an active feedbacksynthesis mode based upon said signal, performing said active currentfeedback comprising using an active feedback network for conditioning anoutput signal, converting said signal into a differential currentsignal, and summing at least two components of said differential currentsignal for performing a differential feedback.
 2. The method of claim 1,wherein receiving said signal further comprises receiving atelecommunications signal.
 3. The method of claim 1, further comprising:providing a first feedback resistor pair that is proportional to a firstoutput sense resistor; providing a second feedback resistor pair inseries with said first feedback resistor, said second feedback resistorbeing proportional to a second output sense resistor; providing afeedback impedance in series with said first and second feedbackresistor pairs, said feedback impedance being proportional to an outputimpedance; and a signal summing node pair to sum said components of saidconditioned differential output signal.
 4. The method of claim 3,wherein performing a signal summation of at least two components of saidconditioned output signal for feedback further comprises generating acanceled differential feedback signal by canceling a first signal from afirst datasense amplifier output that is based upon a first component ofsaid conditioned output signal with a second signal from a seconddatasense amplifier output that is based upon a second component of saidconditioned differential output signal, and performing a differentialfeedback operation using said canceled feedback signal.
 5. An apparatus,comprising: means for receiving a signal; and means for performing ancurrent feedback of an output signal in an active feedback synthesismode based upon said signal, performing said active current feedbacksynthesis comprising using an active feedback network for conditioningan output signal, converting said signal into a differential currentsignal, and summing at least two components of said differential currentsignal for performing a differential feedback.
 6. An apparatus,comprising: a first amplifier and a second amplifier to buffer adifferential input signal to generate a differential output signal; andan active impedance network to condition said differential output signaland sum at a first component and a second component of a first portionof said conditioned differential output signal, and to sum first andsecond components of a second portion of said conditioned differentialoutput signal for generating a differential feedback signal for feedbackinto said first and second amplifiers.
 7. The apparatus of claim 6,further comprising: a first and a second sense resistor pairs fordetecting a current level of said differential input signal; a firstdatasense amplifier operatively coupled with a terminal of said firstsense resistor and a first terminal of said active impedance network,said first datasense amplifier to generate said first portion of saidconditioned output signal; and a second datasense amplifier operativelycoupled with a terminal of said second sense resistor and a secondterminal of said active impedance network, said second datasenseamplifier to generate said second portion of said conditioned outputsignal.
 8. The apparatus of claim 6, wherein said signal summation ofsaid first and said second components of said first and second portionsprovides a canceled differential feedback signal.
 9. The apparatus ofclaim 6, wherein said active impedance network comprises a firstresistor, a feedback impedance unit in series with said first resistor,and a second resistor in series with said feedback impedance unit, saidfirst resistor, second resistor, and said feedback impedance unit eachbeing proportional to an output impedance of said apparatus.
 10. Theapparatus of claim 9, wherein said output impedance is comprised of atransformer.
 11. The apparatus of claim 10, wherein said first resistor,second resistor, and said feedback impedance unit each beingproportional to a turns ratio of said transformer.
 12. The apparatus ofclaim 11, wherein first resistor, second resistor, and said feedbackimpedance unit each being proportional to said output impedance by afactor of
 100. 13. An apparatus, comprising: a voltage-to-current signalconverter to convert a differential input voltage signal; a first bufferamplifier operatively coupled to said voltage to current signalconverter, said first buffer amplifier to receive a component of aconverted differential input signal on a first input terminal andproduce a first buffered current signal; a first sense resistoroperatively coupled with said buffered amplifier; a second bufferamplifier operatively coupled to said voltage-to-current signalconverter, said second buffer amplifier to receive a component of saidconverted differential input signal on a second input terminal andproduce a second buffered current signal; a second sense resistoroperatively coupled with said second buffer amplifier; an activefeedback impedance network operatively coupled with said first andsecond sense resistors; a first datasense amplifier operatively coupledto said active feedback impedance network and to said first senseresistor, said first datasense amplifier to generate a first and secondcomponent of a differential feedback signal; a second datasenseamplifier operatively coupled to said active feedback impedance networkand to said second sense resistor, said second datasense amplifier togenerate a third and fourth component of said differential feedbacksignal; a first summing node for summing said first and secondcomponents of said differential feedback signal to generate a firstcomponent of a summed differential feedback signal for feedback intosaid first buffer amplifier; and a second summing node for summing saidthird and fourth components of said differential feedback signal togenerate a second component of said summed differential feedback signalfor feedback into said second buffer amplifier.
 14. The apparatus ofclaim 13, wherein said summed differential feedback signal is a canceleddifferential feedback signal.
 15. The apparatus of claim 13, whereinsaid active impedance network comprises a first resistor, a feedbackimpedance unit in series with said first resistor, and a second resistorin series with said feedback impedance unit, said first resistor, secondresistor, and said feedback impedance unit each being proportional to anoutput impedance of said apparatus.
 16. The apparatus of claim 15,wherein said output impedance is comprised of a transformer.
 17. Theapparatus of claim 16, wherein said first resistor, second resistor, andsaid feedback impedance unit each being proportional to a turns ratio ofsaid transformer.
 18. The apparatus of claim 17, wherein said turnsratio of said transformer is
 100. 19. A system, comprising: a subscriberline; and a line card electronically coupled with said subscriber line,said line card being adapted to: receiving a signal; and performing acurrent feedback of an output signal in an active feedback synthesismode based upon said signal, performing said active current feedbackcomprising using an active feedback network for conditioning an outputsignal, converting said signal into a differential current signal, andsumming at least two components of said differential current signal forperforming a differential feedback.
 20. The system of claim 19, whereinsaid line card comprising: a first amplifier and a second amplifier tobuffer a differential input signal to generate a differential outputsignal; and an active impedance network to condition said differentialoutput signal and sum at a first component and a second component of afirst portion of said conditioned differential output signal, and to sumfirst and second components of a second portion of said conditioneddifferential output signal for generating a differential feedback signalfor feedback into said first and second amplifiers.
 21. The system ofclaim 20, said line card further comprising: a first and a second senseresistor pairs for detecting a current level of said differential inputsignal; a first datasense amplifier operatively coupled with a terminalof said first sense resistor and a first terminal of said activeimpedance network, said first datasense amplifier to generate said firstportion of said conditioned output signal; and a second datasenseamplifier operatively coupled with a terminal of said second senseresistor and a second terminal of said active impedance network, saidsecond datasense amplifier to generate said second portion of saidconditioned output signal.
 22. The system of claim 21, wherein saidsignal summation of said first and said second components of said firstand second portions provides a canceled differential feedback signal.23. The system of claim 22, wherein said active impedance networkcomprises a first resistor, a feedback impedance unit in series withsaid first resistor, and a second resistor in series with said feedbackimpedance unit, said first resistor, second resistor, and said feedbackimpedance unit each being proportional to an output impedance of saidapparatus.
 24. The system of claim 23, wherein said output impedance iscomprised of a transformer.
 25. The system of claim 24, wherein saidfirst resistor, second resistor, and said feedback impedance unit eachbeing proportional to a turns ratio of said transformer.
 26. The systemof claim 25, wherein said first resistor, second resistor, and saidfeedback impedance unit each being proportional to said output impedanceby a factor of 100.